GAINESVILLE, Ga.—Weeks after his election as Georgia governor in 2010, Nathan Deal was pulled aside by a conservative state lawmaker with urgent business to discuss.

Rep. Jay Neal, a small-town pastor, said he had the seeds of a plan to cut Georgia’s swelling prison population, which was costing taxpayers over $1 billion a year. The governor-elect didn’t let Mr. Neal get far.

“The minute I mentioned what I wanted to do, he jumped in with what he wanted to do,” Mr. Neal recalled. “And it turns out we were talking about the same thing.”

That pairing of a pastor with a former prosecutor, both Republicans, helped pave the way for dramatic revamping of Georgia’s criminal code. New rules enacted over the past two legislative sessions are steering nonviolent offenders away from prison, emphasizing rehabilitation over jail time, and lessening the penalties for many drug and property crimes.

Georgia is the latest example of a Republican-led state drive to replace tough-on-crime dictums of the 1990s with a more forgiving and nuanced set of laws. Leading the charge in states such as Texas, Ohio, Kentucky, South Carolina and South Dakota are GOP lawmakers—and in most cases Republican governors—who once favored stiff prison terms aimed at driving down crime.

Motivations for the push are many. Budget pressures and burgeoning prison costs have spurred new thinking. Some advocates point to data showing that harsh prison sentences often engender more crime. Among the key backers are conservative Christians talking of redemption and libertarians who have come to see the prison system as the embodiment of a heavy-handed state. And crime rates are falling nationally, a trend that has continued in most of the states putting fewer people in jail.

The movement also dovetails with the quest of some Republicans to soften the party’s edges and to plunge into new policy areas that affect the poor and the disadvantaged. The initiatives have drawn praise from groups that aren’t often allied with the GOP, including the National Association for the Advancement of Colored People and the American Civil Liberties Union. The result is some unlikely bedfellows, with the conservative American Legislative Exchange Council working alongside the ACLU.

“Criminal justice is the area where conservative thinking has most changed with the times,” said Eli Lehrer, a former GOP Senate staffer and conservative activist in Washington, who has written extensively on the push for new sentencing rules. He describes the push as “the most important social reform effort on the right since the rise of the pro-life movement in the 1970s.”

Just over half of the states have embarked on criminal-justice overhauls of varying scope over the past five years, with 19 of those efforts led by Republican governors or GOP legislatures and nine by Democratic governors or legislatures. Some of the most aggressive moves have come in states, many in the South, with incarceration rates well above the national average.

The number of inmates in state prisons nationally peaked at just over 1.4 million around 2009 after rising for decades, and by 2011 had fallen by about 25,000, according to Justice Department statistics.

The downturn has been particularly welcome in states that had projected a continued surge in prison numbers. Ohio, which was bracing for an inmate population of over 57,000 by the end of the decade, has seen its number fall by nearly 1% a year since 2009.

Changes to sentencing laws haven’t sailed everywhere. In Indiana, an aggressive push in 2011 by then Republican Gov. Mitch Daniels got watered down—and eventually abandoned—after it ran into opposition from prosecutors. GOP Gov. Rick Scott in Florida cited public safety last year when he vetoed a bill to cut the sentences for nonviolent drug offenders.

The conservative quest to rethink criminal sentencing and rewrite state penal codes got its start in Texas, when GOP lawmakers in 2007 balked at the need to build three new prisons to house an anticipated 17,000 more prisoners by 2012. They decided instead to revamp the state’s probation system and boost funding for addiction treatment and rehabilitation by $241 million.

The state prison population has declined by nearly 6,000 inmates since 2008 after decades of rapid growth and during a time when the state’s own population has continued to swell. In 2011, Texas shut a prison for the first time in state history.

Behind the Texas efforts stood a conservative local think tank, the Texas Public Policy Foundation, and one of its top donors, a wealthy oil man from Odessa named Tim Dunn. Mr. Dunn paid to establish a center within the foundation in 2005 to focus on overhauling the state’s criminal code.

An evangelical Christian with a strong libertarian bent, Mr. Dunn said he watched for years as Texas’ crime rate continued to climb even while its prison population swelled. “I had come to see our justice system as imperial, as intent on maintaining the authority of the king. It was no longer communal or restorative,” he said.

Under the directorship of Texas lawyer Marc Levin, the policy foundation became the hub of a national movement as requests for legislative help poured in from other states. The center adopted a formal platform in early 2010 and took its campaign national under the name Right on Crime.

It soon had the backing of a long list of conservative supporters, among them former Florida Gov. Jeb Bush, former Reagan Attorney General Ed Meese, former drug czar Bill Bennett and David Keene, until recently president of the National Rifle Association.

The group and its Republican followers are sensitive to charges that they are going soft on crime, “that we want to hug a thug,” as Mr. Dunn puts it.

But they insist they are moving to correct a system that tilted too far toward punishment, without any gauge for success or failure. State prison populations swelled 700% between 1970 and 2009, from 174,000 inmates to 1.4 million.

Legislatures across the country have rewritten their criminal-justice codes. A few Democratic governors have jumped in, including Arkansas’s Mike Beebe and Hawaii’s Neil Ambercrombie. New York and Connecticut made changes even before Texas did.

But “on balance, it has been conservatives who have been out front,” said Adam Gelb, who directs a national criminal-justice initiative at the Pew Charitable Trusts, which has worked on initiatives across the country.

In many states, former law-and-order prosecutors and judges have led the effort. In others, pastors-turned-lawmakers have jumped in. Many describe eureka moments that altered their views.

In Pennsylvania, state Sen. Stewart Greenleaf, a former prosecutor, helped pass many of the state’s toughest sentencing laws in the 1980s. The Republican lawmaker then watched as both the crime rate and the prison population continued to soar. He is now the leading force in the state to promote alternatives to prison, boost rehabilitation programs and soften the rules on probation violations—all of which have been put into effect.

For Ohio Republican state Sen. Bill Seitz, a turning point came in the late 2000s, when he watched the voters in his county, which includes Cincinnati, twice vote down levies to build a new jail. “It became all the clearer to me how we pass tough sentencing laws with a blind eye to the fiscal impacts,” he said.

He has since successfully championed legislation in Ohio to steer new nonviolent felons away from prison, to speed the release of some who are already locked up and to make it easier for them to erase their criminal record and find work when they get out.

As a result, Ohio’s prison population dropped to 49,700 inmates at the end of last year from a peak of 51,278 in 2008.

In Georgia, Gov. Deal and Rep. Neal arrived at their partnership via similar and very personal paths.

Mr. Deal says his evolution came about largely on the streets of his hometown of Gainesville, an hour’s drive north of Atlanta. For nearly a decade, his son Jason has presided over a drug court designed to rehabilitate addicts charged with felonies and to keep them out of prison.

The future governor often went to graduation ceremonies where recovering addicts would tell their stories. “They all have their own stories, but a common thread runs through all of them,” Gov. Deal said. “They had lied. They had stolen. They had alienated their spouses, their parents, their siblings. But they were given a second chance, and they had been rehabilitated.”

As a pastor, Mr. Neal came to know recovering addicts in his church. In 2005, he guided into law a measure to crack down on methamphetamine labs, which were plaguing his corner of northeastern Georgia. At the urging of the Georgia Council on Substance Abuse, he later went to a seminar in Philadelphia on the science of drug addiction.

“That was my real ah-ha moment,” he said. “I realized there are so many factors in people’s lives we can’t simply punish away.”

The governor never mentioned a sentencing overhaul during his 2010 campaign. But he had mulled the issue privately, and he agreed to join forces with Rep. Neal and other lawmakers to make a major push in his first term.

He announced his intentions in his first speech before the state legislature in January 2011. “We cannot afford to have so many of our citizens waste their lives because of addictions,” he said. “It is draining our state treasury and depleting our workforce.”

Georgia at the time had the country’s highest rate of people caught up in the criminal justice system, according to Pew, with one in 13 of its residents behind bars, on probation or on parole. The tab to the state: $3 million a day.

The state passed laws to steer nonviolent criminals away from prison, to give judges wider discretion in sentencing and to make it easier for defendants to seek rehabilitation services. The governor also put $10 million a year into expanding so-called accountability courts, such as the drug court his son, Jason Deal, presides over in Hall and Dawson counties. The number of such courts has nearly tripled in Georgia under Gov. Deal, to 247, compared with 87 in December 2010.

In Gainesville, 427 would-be felons have graduated from Judge Deal’s drug court since it began nearly a decade ago. Each went through a two-year program of mandatory employment or schooling, frequent drug tests and group counseling. The program costs around $13 a day per person, compared with $50 a day to feed and house a state prisoner. After their release, nearly a third of state prisoners end up committing another crime. The recidivism rate among drug-court graduates is just 8%, a recent state audit found.

Jennie Mercado is on course to graduate soon from Judge Deal’s drug court. Arrested two years ago on multiple felony drug counts, the 27-year-old Texas native said she went to church the day before her arrest to pray that some force would step in to alter her life.

“I was a total full-time junkie and thief for 10 years,” she said. Once facing a year in prison, she is now training to be a nurse. She gave birth to a daughter three months ago.

She now speaks of her arrest and trip through drug court as a stroke of luck. “Nothing but great stuff has come out of this,” she said.

Supporters of the changes in Georgia and other states note that elected officials such as Gov. Deal have done little to publicize their efforts, much less campaign on them.

Ben Jealous, president of the NAACP, sees that as a missed opportunity. “This is an area where Republicans can really connect with black voters,” he said.

Gov. Deal acknowledges there are risks in championing prison changes. “You always worry about being accused of being soft on crime,” he said. But through a spokesman he said he now “very much wants to be seen as the face of prison reform in this state.”